1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine stator vane in a large industrial engine with purge air for a rim cavity.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
The turbine includes stages or rows of stator vanes and rotor blades with labyrinth seals formed between a rotating part and a static part to prevent hot gas ingestion from the main hot gas stream into an inter-stage housing. The rotor disks are much more temperature sensitive than the blades and vanes, and an over-temperature of the rotor disk can lead to premature cracking and thus rotor disk destruction. Thus, the need for purge air into the forward and aft rim cavities to prevent excess hot gas ingestion.
FIG. 1 shows a side view of a prior art turbine stator vane with an airfoil extending between an outer diameter endwall and an inner diameter endwall. FIG. 2 shows a top view of the serpentine flow cooling circuit of the prior art vane and includes a first leg or channel 11 along the leading edge of the airfoil, a second leg 12 and then a third leg 13 all connected in series. The third leg feeds cooling air to a row of exit holes 15 located along the trailing edge of the airfoil. Trip strips are used along the side walls of the legs to enhance heat transfer from the hot metal to the cooling air.
FIG. 3 shows a flow diagram for the prior art vane cooling circuit with the first leg 11 flowing into an inner endwall turn channel, then into the second leg and into an upper endwall turn channel, and then into the third leg and out through the exit holes 15 or the aft rim cavity purge hole 17 at the end of the third leg 13. A front rim cavity purge hole 16 is connected to the inner endwall turn channel to supply purge air to a front rim cavity.
FIGS. 5 and 6 shows prior art arrangements for purge air holes used for the front rim cavity. In both of these designs, a pressure loss in the cooling air occurs because of the path the cooling air travels for the serpentine flow circuit and for the purge air. The FIG. 5 design produces not only a large pressure drop but forms a stagnation zone along the lower end of the leading edge that results in low cooling air flow at this surface and causes a hot metal spot that leads to erosion. The FIG. 6 design produces a lower pressure loss than the FIG. 5 design, but also produces a pressure loss in the purge air flowing through the purge air hole on the bottom of the turn channel.